The present invention relates to a light emitting device and a manufacturing method thereof and a display using this light emitting device, in particular, which is an organic electroluminescent (EL) device that emits light having wide range wavelengths from blue to red.
Description of the Related Art
An organic EL device is a self light emitting device, which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode.
Since a report on a low-voltage-driven organic EL device employing multilayered structure was released by C. W. Tang et al. of Eastman Kodak Co. (C. W. Tang, S. A. VanSlyke, Applied Physics Letters, Vol. 51, page 913 (1987)), extensive researches have been made on organic EL devices, that is, EL devices employing organic materials.
In the above report, an organic EL device employing tris(8-quinolinol)aluminum complex for the light-emitting layer and triphenyldiamine derivative for the hole transporting layer was fabricated on a glass substrate. The multilayer structure has some advantages such as: improved hole injection to the light-emitting layer; increase of production efficiency of excitons which are generated by hole-electron recombination (by blocking the paths of electrons injected from the cathode); and confinement of the excitons generated in the light-emitting layer.
As the structure of the organic EL devices, two-layer types (including a hole transporting (and injection) layer and an electron transporting light-emitting layer) and three-layer types (including a hole transporting (and injection) layer, a light-emitting layer and an electron transporting layer) are well known. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
Further, organic EL devices involve certain limitations on the probability of the creation of singlet excited states of light-emitting material molecules on carrier recombination since the carrier recombination is dependent on spin statistics, thereby the probability of light emission is necessitated to have an upper limit. The upper limit is known as approximately 25%. FIG. 1 is a sectional view of a conventional organic EL device. In FIG. 1, the conventional organic EL device has a structure in which an anode 2, a luminescent layer 3, and a cathode 4 are layered on a substrate 1. As shown in FIG. 1, in conventional organic EL devices, rays of light whose outgoing angles (getting out of the light-emitting layer) are larger than a critical angle (depending on the refractive index of the light-emitting material) can not get out of the light-emitting layer due to total reflection. Therefore, when the refractive index of the light-emitting material is 1.6, only about 20% of the total light emission is available outside, and the upper limit of energy conversion efficiency becomes as low as approximately 5% taking the singlet excited states creation probability into account (Tetsuo Tsutsui “Present situation and trends in organic electroluminescence”, Display (monthly), vol.1, No.3, page 11 (September 1995). In organic EL devices, having tight limitations on the light emission probability, low light extraction efficiency (low efficiency in extracting light from the organic EL device to outside) causes fatal deterioration of the (total) luminescent efficiency.
In order to make the light extracting efficiency higher, many methods have been studied at for light emitting devices such as a conventional inorganic EL device having a similar structure to the organic EL device. For example, Japanese Patent Application Laid-Open No. SHO 63-314795 discloses a thin film EL device. In this patent application, the efficiency is made to be high by forming or attaching light convergent optics on the substrate. And Japanese Patent Application Laid-Open No. HEI 1-220394 discloses a high luminance EL device. In this patent application, the efficiency is made to be high by forming a reflection mirror on the side of the device. These methods are effective for a device having a large luminescent area. However, for a device whose pixel area is small, such as a dot matrix display, it is difficult to form the lenses for light convergence or form the reflection mirror on the side of the device. Further, at the organic EL device, the thickness of the luminescent layer is less than a few μm, therefore, it is difficult to form the reflection mirror at the side of the device by tapering the side at the current fine processing technology. If it is realized, the cost is increased largely.
Japanese Patent Application Laid-Open No. SHO 62-172691 discloses a thin film EL device. In this patent application, a flattened layer, whose refractive index is between a glass substrate and a luminescent layer, is disposed between the glass substrate and the luminescent layer, and this flattened layer is made to be an anti-reflection layer. This method has an effect to improve the light extracting efficiency to the forward, but cannot prevent the total internal reflection. Therefore, while being effective for inorganic EL devices (including materials with large refractive indices), the method can not effectively improve the light extraction efficiency of organic EL devices (including light-emitting materials of relatively low refractive indices).
In case that the organic EL devices are used in a display, the organic EL devices are required to emit multi color. There are following three methods to make the organic EL devices emit multi color.
At the first method, the organic EL device is formed by using one of different luminescent materials for each color, or by doping one of different fluorescent dye for each color. At the second method, light having necessary luminescent color is obtained by eliminating light of unnecessary color from the luminescent of the organic EL element, in which light of necessary colors are included, by using color filters (color filter method). At the third method, light of a color is extracted through a filter (light conversion layer), which contains a fluorescent dye which absorbs the light emitted from organic EL element and emits light of the color (light conversion method).
For a full color display used for displaying such as a natural picture, pixels of R, G, and B are arrayed. Examples having RGB pixels which are formed by using the first method are disclosed in Japanese Patent Applications Laid-Open No. HEI 5-275172, HEI 5-258859, and HEI 5-258860. In these applications, it is necessary that each pixel for the RGB is formed separately, therefore, the forming process is very complicated, and it is disadvantageous to its mass production.
At the second or the third method, a color filter applied patterning or the light conversion layer applied patterning is used. Therefore, patterning for the organic EL layer is not necessary; consequently, a full color display is easily realized. An example of a full color display realized by using the second method is disclosed in Japanese Patent Application Laid-Open No. HEI 7-220871. In this patent application, color filters are combined with the organic EL device emitting white light. And examples, of a full color display realized by using the third method, are disclosed in Japanese Patent Applications Laid-Open No. HEI 3-152897 and HEI 11-121164.
However, several repeats of the photo lithography process are required for forming the color filters or the light conversion layer and the process becomes quite complex. Further, the surface of them becomes uneven. In case that the organic EL device is disposed at a position close to or very close to the color filters or the light conversion layer, even when flattening is applied by forming a flattened layer, a short-circuit of pixels frequently occurs. Further, in order to avoid the influence from chemical compounds outputting from the color filters or the light conversion layer, a protection layer must be inserted. Consequently, the performance of the organic EL device is decreased and the manufacturing cost is increased, that is, there are many problems at its mass production.
In order to solve these problems, there is a method in which the color filters or the light conversion layer is disposed at a position having a distance from the organic EL layer such as at the opposite side of the substrate. However, in this method, light emitted by an organic EL device (corresponding to a pixel area) tends to reach adjacent pixel areas, thereby the light leakage problem (light emission from pixel areas that are not supposed to emit light) occurs. In order to resolve the light leakage problem, Japanese Patent Application Laid-Open No.HEI11-8070 disclosed a method, in which a black mask and a light diffusing layer were formed between the substrate and the organic EL devices. However, the method further deteriorates the light extraction efficiency, since part of the light emitted by the organic EL devices is absorbed by the black mask.
As described above, the prevention of light leakage and the improvement of light extraction efficiency in light-emitting devices employing organic EL devices are both still insufficient. Therefore, techniques capable of satisfying both of the requirements are being sought for, and the development of such techniques is essential for practical utilization of the organic EL devices.